可以说
sb. is happy to do sth. 什么人乐于做什么事
We are happy to help you. 我们乐于(愿意)帮助你
sb. is happy with sth. 什么人乐于什么事
I am happy with my exam scores. 我对我的考试成绩满意
sb. is happy in doing sth. 什么人乐于做什么事
He is happy in helping others. 他乐于帮助他人
可以说
sb. is happy to do sth. 什么人乐于做什么事
We are happy to help you. 我们乐于(愿意)帮助你
sb. is happy with sth. 什么人乐于什么事
I am happy with my exam scores. 我对我的考试成绩满意
sb. is happy in doing sth. 什么人乐于做什么事
He is happy in helping others. 他乐于帮助他人